Method and Composition For Sorbing Toxic Substances

ABSTRACT

Toxic substances such as heavy metals are extracted from a medium using a sorbent composition. The sorbent composition is derived by sulfidation of red mud, which contains hydrated ferric oxides derived from the Bayer processing of bauxite ores. Exemplary sulfidizing compounds are H 2 S, Na 2 S, K 2 S, (NH 4 ) 2 S, and CaS x . The sulfur content typically is from about 0.2 to about 10% above the residual sulfur in the red mud. Sulfidized red mud is an improved sorbent compared to red mud for most of the heavy metals tested (Hg, Cr, Pb, Cu, Zn, Cd, Se, Th, and U). Unlike red mud, sulfidized red mud does not leach naturally contained metals. Sulfidized red mud also prevents leaching of metals when mixed with red mud. Mixtures of sulfidized red mud and red mud are more effective for sorbing other ions, such as As, Co, Mn, and Sr, than sulfidized red mud alone.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/537,907,filed Aug. 7, 2009, which is a division of U.S. application Ser. No.11/277,282, filed Mar. 23, 2006, the disclosures of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention is directed to sorbents for heavy metals and theiruse for facile extraction of heavy metals from liquid and gaseousstreams, as well as removal of the sorbed heavy metals by solids/fluidseparation means.

DESCRIPTION OF RELATED ART

Heavy metal contaminated flue gases and liquids from various sources(ground, stream, runoff, mines, petroleum, industrial waste) are amongthe most dangerous and difficult environmental problems facing the worldtoday. An especially serious problem is posed by toxic metals in suchstreams. Among these metals are mercury, chromium, cobalt, nickel,copper; zinc, silver, gold, cadmium, lead, selenium, and transuranicelements.

Mercury contamination of the environment is the subject of increasingattention because it eventually accumulates at very high levels in thebodies of large predatory fish such as tuna, swordfish, and sharks. Amajor concern is the atmospheric release of mercury from coal firedpower plants, currently estimated at 46 tons per year in the UnitedStates. The Environmental Protection Agency (EPA) has identified womenof childbearing age as especially threatened because of possibleneurological damage to unborn children. It is estimated that 8% of womenin this category have a methyl mercury blood level above 5.8 ppb.

On Dec. 14, 2000, the EPA issued a determination that their agency mustpropose new regulations under the Clean Air Act to control mercuryemissions from coal and oil fined power plants by Dec. 15, 2003. Oneproposal was to reduce mercury emissions from power plants by 90% by2007. According to an article in Forbes (Apr. 14, 2003, page 104) suchregulation “could cost the power industry at least 8.8 billion dollarsper year.” Other, more recent proposals such as the Clear Skies Act callfor a 70% reduction in mercury emissions over 15 years.

At present, a major control technology for mercury is the use ofactivated carbon treatment of flue gases from power plants. Activatedcarbon currently sells for about 45 cents per pound ($ 900 per ton) butthe disposal or possible regeneration of mercury-sorbed activated carbonpresents unresolved problems at this time.

Red mud is an undesirable by-product and major pollutant from the BayerProcess. Bayer caustic leaching of bauxite is the principal process forproduction of alumina. This process relies on the solubility ofaluminous minerals in hot (e.g., 125-250° C.) sodium hydroxide solutionand the insolubility of most of the remaining minerals (iron, titaniumcompounds and silica), which are either insoluble or react andre-precipitate. The insoluble, iron rich residue byproduct is known as“red mud.” Red mud can contain from about 17.4 to 37.5% iron (Fe)(Bauxite Residue Fractionation with Magnetic Separators, D. WilliamTedder, chapter 33, Bauxite symposium, 1984, AIME 1984). Red mud is acomplex mixture of finely divided hydrated iron oxides with a widevariety of lesser minerals (Al, Na, Ti, Si, Ca, Mg) and traces of over ascore of other elements (Cr, Ni, Zn, Pb, As, etc). These hydrous ironoxides have extraordinary sorptive and complexing properties.

Red mud is a very hydrophilic, high pH slime which is extremelydifficult to dewater by filtration or sedimentation means. Thiscomplicates and limits its utility as a sorbent in aqueous systems.

Red mud has been proposed as a sorbent for heavy metals, cyanides,phosphates, and the like (David McConchie, Virotec website:virotec.com/global.htm). However, the sorptive and release properties ofred mud are not always complementary. Depending on the source of aparticular red mud, it can also leach out significant amounts of toxicpollutants such as radioactive thorium, uranium, chromium, barium,arsenic, copper, zinc, cobalt, as well as lead, cadmium, beryllium, andfluorides.

The potential problems involved with use of red mud to control pollutionare highlighted in an e-newsletter article entitled “The Great Red MudExperiment that Went Radioactive”—Gerard Ryle, May 7, 2002(smh.com.au/articles/2002/05/06/1019441476548.html). This experimentconducted by the Western Australian Agricultural Department involvedplacing 20 tonnes of Alcoa red mud per hectare on farmland in order tostop unwanted phosphorous from entering waterways. An unintended resultof this application was that runoff waters showed excessive quantitiesof copper, lead, mercury, arsenic, and selenium. Emaciated cattlegrazing on such land exhibited high chromium, cadmium, and fluoridelevels. Each hectare contained up to 30 kilograms of radioactivethorium. The disastrous red mud application test was abruptly terminatedafter five years.

It is therefore evident that extreme caution must be exercised inselecting and testing red mud before attempting to use it to sorb toxiccompounds.

Furthermore, the capacity of red mud to capture and hold toxicsubstances such as mercury and related metals is not adequate toeliminate traces of these metals in leachate. The possibility alsoexists that sorption of one toxic pollutant may release otherpollutants. As a result, use of red mud as a sorbent to achieve drinkingwater standards can be problematic.

There remains a need for improved sorbents for extracting toxiccompounds such as mercury and other heavy metals.

SUMMARY OF THE INVENTION

The present invention, according to one aspect, is directed to a sorbentcomprising the reaction product of a sulfidizing compound and red mud.Red mud contains hydrated ferric oxides derived from Bayer processing ofbauxitic ores. The sorbent is particularly useful for sorbing toxicsubstances from a medium, such as heavy metals present in a liquid orgaseous stream. Exemplary sulfidizing compounds include H₂S, Na₂S, K₂S,(NH₄)₂S, and CaS_(x). The sulfur content of the reaction producttypically is from about 0.2 to about 10% above the residual sulfur inthe red mud.

According to one aspect of the invention, potable water (e.g., meetingdrinking water standards) is prepared by treating contaminated waterwith a sulfidized red mud sorbent.

According to another aspect of the invention, heavy metals such asmercury are sorbed from flue gases of coal- or oil-fired power plants bytreating the flue gases with a sulfidized red mud sorbent.

According to another aspect of the invention, heavy metals are sorbedfrom mine drainage waters by treating the mine drainage waters with asulfidized red mud sorbent.

According to yet another aspect of the invention, heavy metals aresorbed from a hydrocarbon stream, such as a petroleum stream, bytreating the stream with a sulfidized red mud sorbent.

The sorbent of the present invention is effective for sorbing variouscontaminants, such as mercury, which are not effectively sorbed by redmud. Conversely, red mud is effective for sorbing other contaminants,such as arsenic, which are not effectively sorbed by the sulfidized redmud sorbent. Thus, some treatments can benefit by using both red mud andsulfidized red mud, either in the same sorbent composition or inseparate treatment stages. Such sorbent combinations potentially canallow for the extraction of a wider range of contaminants.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has applicability in removing contaminants from awide variety of mediums, non-limiting examples of which include fluegases and liquids from various sources such as groundwater, waterstreams, runoff, mines, petroleum streams, and industrial waste streams.Of particular interest is sorbing heavy metals, such as mercury (Hg),chromium (Cr), lead (Pb), copper (Cu), zinc (Zn), silver (Ag), gold(Au), cadmium (Cd), selenium (Se), thorium (Th), and uranium (U), fromsuch mediums. The metal(s) may be present as ions, as free elements, orin compounds with other elements.

The sorbents of the present invention can be used for the preparation ofpotable water, e.g., meeting drinking water standards. Other exemplaryapplications include sorbing heavy metals, such as mercury, from fluegases of coal- or oil-fired power plants, mine drainage waters, orhydrocarbon streams such as petroleum streams.

The sorbent can be prepared by the sulfidation of red mud, whichcontains hydrated ferric oxides derived from the Bayer processing ofbauxitic ores. Sulfidation can be achieved by reacting the red mud withone or more sulfidizing compounds such as H₂₅, Na₂S, K₂S, (NH₄)₂S, andCaS_(x). Unlike red mud, which is very hydrophilic, the sulfidized redmud is lyophobic and more readily dewatered. As a result, sulfidized redmud exhibits significantly faster filtration rates than those exhibitedby red mud.

The relative amount of the sulfidizing compound preferably is selectedso that the sulfur content of the reaction product is from about 0.2 toabout 10% above the residual sulfur content of the red mud. The weightratio of sulfidizing compound to red mud will vary on the type ofsulfidizing compound used and the desired level of sulfidation for aparticular end use. Most often, the sulfidizing compound and red mud arecombined at a weight ratio of from about 1:40 to about 1:4, more usuallyfrom about 1:25 to about 1:6, and even more usually from about 1:20 toabout 1:8.

The conditions under which the red mud can be sulfidized depend on suchfactors as the identity of the sulfidizing compound(s) and the intendeduse of the resulting sorbent. In some cases, sulfidation can beaccomplished by mixing red mud and the sulfidizing compound at ambienttemperature and atmospheric pressure. In general, higher sulfur contentscan be obtained when the reaction is carried out at elevatedtemperatures and/or elevated pressures. Sulfur content in the reactionproduct also can be influenced by factors such as the sulfur content ofthe sulfidizing agent. For example, compounds with higher sulfurcontents, such as calcium polysulfide, typically yield products havinghigher sulfur contents.

When using gaseous sulfidizing compounds, such as hydrogen sulfide(H₂S), it is often preferable to conduct the reaction at elevatedtemperature and/or elevated pressure to increase the rate of reactionand the sulfur content of the resulting sorbent. Suitable exemplaryreaction temperatures range from about 40 to about 200° C., often fromabout 80 to about 120° C. The reaction pressure typically ranges fromabout 1 to about 225 psi, often from about 30 to about 70 psi(absolute).

In one embodiment of the present invention, the sorbent is slurriedtogether with the medium containing the contaminant(s) to be extracted.Suitable mixing equipment can be used to provide sufficient contactbetween the sorbent and the contaminant(s). The sorbent, which forms acomplex with the contaminant(s), can then be separated from the slurryusing one or more conventional techniques such as filtration,sedimentation, or centrifugation.

In an alternative embodiment of the present invention, the sulfidizedred mud sorbent is processed into pellets or the like using conventionalpelletizing or extrusion equipment. Preparing the sorbent in pellet formcan simplify its handling and/or use. The pellets may be incorporatedinto filters of conventional construction for use in a variety ofindustrial or consumer filtration applications, such as filters usablefor preparing potable water.

It has been found that the sulfidized red mud sorbent is effective forsorbing various contaminants, such as mercury, which are not effectivelysorbed by red mud. On the other hand, red mud is effective for sorbingother contaminants, such as arsenic, which are not effectively sorbed bysulfidized red mud. For the treatment of mediums having contaminants inboth of these categories, the use of red mud and sulfidized red mud intandem, either in the same sorbent composition or in sequentialtreatment stages (e.g., red mud followed by sulfidized red mud) can bemore effective than using either sorbent alone.

EXAMPLES Example 1

This example shows the preparation of red mud. A 1 kg sample of red mudreceived from Sherwin Alumina Company of Corpus Christi, Tex. wasslurried at 15% solids in demineralized water and filtered on a Buchnerfunnel. The resulting filter cake was re-slurried with demineralizedwater, re-filtered, and used as the starting material in Example 2.

Example 2

This example illustrates the preparation of sulfidized red mud usinghydrogen sulfide (H₂S). Washed red mud (100 g) from Example 1 wasslurried in demineralized water at 15% solids and the stirred slurry wassaturated with hydrogen sulfide for 30 minutes at ambient temperature.The sample was dried overnight at 100° C. and the resulting cake waspulverized.

Example 3

This example shows the preparation of sulfidized red mud using H₂S underpressure in a Parr Bomb. The sulfidation procedure of Example 2 wasrepeated using a Laboratory Parr Bomb. After saturation of the slurrywith hydrogen sulfide gas, the bomb was sealed and heated four hours at100° C. while stirred. The bomb was then cooled, depressurized and thecontents filtered, dried, and pulverized.

Example 4

This example illustrates the preparation of sulfidized red mud usingammonium sulfide (NH₄)₂S. Red mud (200 g) was dispersed in 600 grams ofdeionized (DI) water in a Waring Blender for 5 minutes. Ammonium sulfide(10 g) was added and the slurry was heated with stirring on a hot platefor 1 hr. at 60° C. It was then filtered and dried at 90° C.

Example 5

This example shows the preparation of sulfidized red mud using sodiumsulfide (Na₂S). The procedure of Example 2 was repeated using sodiumsulfide instead of ammonium sulfide.

Example 6

This example illustrates the preparation of sulfidized red mud usingcalcium polysulfide (CaS_(x)). The procedure of Example 2 was repeatedusing 33.5 g of 30% solution of Cascade calcium polysulfide.

Example 7

The following table summaries the sulfur content of the red mud (RM) ofExample 1 and the sulfidized red mud (SRM) of Examples 2, 3, 4, 5, and6.

Code Description Example S (wt %) RM Red Mud 1 0.19 SRM-2 Sulfidized RedMud H₂S 2 0.48 SRM-3 Sulfidized Red Mud H₂S w/Pressure 3 0.90 SRM-4Sulfidized Red Mud (NH₄)₂S 4 0.46 SRM-5 Sulfidized Red Mud Na₂S 5 0.62SRM-6 Sulfidized Red Mud CaS_(x) 6 1.19

A complete analysis of RM, SRM-3, SRM-4, SRM-5, SRM-6 is given in TableA below. The analysis reveals that filtration and washing duringpreparation of sulfidized red mud extracts sodium chloride (except forSRM-5) and reduces bound water in the red mud. It is notable that verysmall amounts of reacted sulfur have such a profound effect on thechemical and physical properties of red mud.

TABLE A Weight % Code Description Na₂O MgO Al₂O₃ SiO₂ P₂O₅ S Cl K₂O CoOTiO₂ MnO Fe₂O₃ BaO RM Control 4.73 0.12 17.1 8.23 1.14 0.19 0.20 0.066.79 6.12 0.73 39.9 0.02 SRM-3 H₂S (b) 3.94 0.14 14.6 9.14 1.38 0.900.11 0.05 6.36 6.79 0.90 46.2 0.02 SRM-4 (NH₄)₂S 4.39 0.13 17.9 9.241.26 0.46 0.15 0.04 8.82 6.95 0.85 42.3 0.02 SRM-5 Na₂S 5.20 0.11 17.28.56 1.15 0.62 0.15 0.03 7.53 6.22 0.75 41.5 0.02 SRM-6 CaS_(x) 4.440.09 16.2 8.41 1.29 1.19 0.14 0.04 9.32 6.60 0.81 41.2 0.02 PPM CodeDescription V Cr Co Ni W Cu Zn As Sn RM Control 1100 1258 99 680 16 119416 47 247 SRM-3 H₂S (b) 1252 1506 121 860 23 138 458 44 177 SRM-4(NH₄)₂S 1093 1379 120 762 30 146 648 46 155 SRM-5 Na₂S 942 1272 103 69524 130 504 31 181 SRM-6 CaS_(x) 1054 1364 113 780 29 138 471 49 155 PPMCode Description Pb Mo Sr U Th Nb Zr Rb Y RM Control 144 <10 424 65 186188 1757 24 673 SRM-3 H₂S (b) 180 <10 498 57 199 207 1503 21 831 SRM-4(NH₄)₂S 176 13 447 36 159 153 1888 <10 748 SRM-5 Na₂S 159 11 387 39 123148 1659 <10 695 SRM-6 CaS_(x) 165 13 431 50 146 146 1767 <10 745

Example 8

This example illustrates leaching of red mud and sulfidized red mud. Inpart (a), a slurry of red mud (50 g) and demineralized water (450 ml)was prepared, mixed for 30 minutes, and filtered. The filtrate wasacidified with 2 ml concentrated nitric acid and analyzed by ICP usingEPA3050 and EPA6010 methods.

In part (b), the procedure of part (a) was repeated using sulfidized redmud from Example 2.

Results are given in Table I and show that leachate from sulfidized redmud (SRM) gave a lower content of heavy metals (low parts per billion)than leachate from the red mud (RM) in every case except Cd, where thedifference was insignificant.

TABLE I Metal Concentration in Leachate (ppm) Hg As Cd Cr Pb Se SRM0.0026 ND* 0.0013 0.0044 ND ND RM 0.0032 0.096 ND 0.0510 0.0064 0.017*ND—Not detectable, below limits

Example 9

This example shows mercuric ion (3.5 ppm) sorption by sulfidized redmud. Ten grams of sulfidized red mud from Example 3 was slurried 30minutes with 1 kg demineralized water containing 3.5 ppm mercury (5.66ppm mercuric nitrate). The slurry was filtered and analyzed for mercury(Hg⁺⁺) by ICP (Method EOA 245.1).

Example 10

This example illustrates mercuric ion (3.5 ppm) sorption by red mud.Example 9 was repeated using red mud.

Example 11

This example shows mercuric ion (22 ppm) sorption by sulfidized red mud.Example 9 was repeated using 22 ppm mercury (Hg⁺⁺).

Example 12

This example illustrates mercuric ion (22 ppm) sorption by red mud.Example 11 was repeated using red mud.

Example 13

This example shows mercuric ion (41 ppm) sorption by sulfidized red mud.Example 9 was repeated using 41 ppm mercury (Hg⁺⁺).

Example 14

This example illustrates mercuric ion (41 ppm) sorption by red mud.Example 13 was repeated using red mud.

Results of Examples 9-14 are summarized in Table II and demonstrate thesuperior performance of sulfidized red mud compared to red mud forsorption of mercuric ion from aqueous solutions.

TABLE II Mercuric Concentration Example In Filtrate Sorbent Control  3.5ppm none  9 0.56 ppm red mud 10  0.2 ppm sulfidized red mud Control 22.0ppm none 11  8.0 ppm red mud 12 0.22 ppm sulfidized red mud Control 41.0ppm none 13 23.4 ppm red mud 14 0.04 ppm sulfidized red mud

Example 15

This example shows mercury (metal) sorption from vapor phase bysulfidized red mud and by red mud (spray absorbed). In part (a), onegram of mercury metal was placed in a two necked round bottom (RB) flaskon a supported heating mantle. One neck of the flask was open and thesecond neck was connected with a Teflon® tube to an aperture in theinlet duct of a spray dryer. The mercury was heated to 300° C. while hotair was aspirated through the vessel. Mercury vapor was entrained in theair as it was drawn into the inlet air duct of the spray dryer heated to300° C. A slurry of 50 g SRM (Example 3) in 450 ml demineralized waterwas sprayed by a rotary atomizer operating at 30,000 rpm. The feed rateof SRM was regulated to produce an outlet temperature of 100° C. fromthe dryer.

In part (b), the procedure of part (a) was repeated using RM (Example 1)instead of SRM.

The mercury content of the spray dried SRM from part (a) and the RM frompart (b) are tabulated in Table III and demonstrate that the SRM had asignificantly improved sorption of mercury.

TABLE III Hg Concentration (ppm) 15(a) SRM-3 61.0 15(b) RM-1 8.1

SRM-3 absorbed 7.5 times as much mercury as RM-1 when spray dried at300° C. inlet and 100° C. outlet in the presence of an air streamcontacted by mercury heated to 250° C. Sulfidized red mud issignificantly superior to red mud as a sorbent for elemental mercurymetal vapor.

Example 16

This example shows mercury (metal) sorption from vapor phase bysulfidized red mud and by red mud (spray absorbed). Example 15 wasrepeated except that a slurry of 100 g SRM in 900 ml demineralized waterwas used. On completion of drying, a 50 g sample (a) was set aside foranalysis and 50 g was re-slurried in 450 ml demineralized water andre-dried (b). Samples 16a and 16b were analyzed for mercury.

This experiment was then repeated using 100 g RM to furnish samples 16cand 16d, which were analyzed. The results of parts (a)-(d) are shown inTable IV below.

TABLE IV Hg Concentration (ppm) 16(a) SRM-3 1^(st) pass 95 16(b) SRM-32^(nd) pass 340 16(c) RM-1 1^(st) pass 43 16(d) RM-1 2^(nd) pass 48

As evident from Table IV, SRM-3 was about twice as efficient as RM-1 onthe 1^(st) pass and about seven times as efficient as RM-1 on the secondpass. The results show that the affinity of SRM-3 for mercury improveswith increased exposure to mercury, indicating an induction effect.

Example 17

This example illustrates mercury (metal) sorption from vapor phase bysulfidized red mud (a) and red mud (b) using a column. In part (a), onegram of mercury was placed in a two necked RB flask supported on aheating mantle. One neck of the flask was open (vented) and the secondneck was connected to a vertical tube 20 cm long and 2.5 cm diameterhalf filled with spray dried sulfidized red mud. A slight vacuum wasapplied to the open end of the packed tube and regulated to fluidize thespray dried sulfidized red mud while the mercury in the flask was heatedto 300° C. The aspiration was continued for 20 minutes, the tube wasdisconnected from the RB flask and the sulfidized red mud contentsanalyzed for mercury by ICP.

In part (b), the procedure of part (a) was repeated using spray driedred mud, after which the red mud was also submitted for mercury analysisby ICP.

Results of the above experiment are tabulated in Table V and demonstrateincreased sorption of mercury vapor by sulfidized red mud (SRM-3)compared to red mud (RM-1).

TABLE V Hg Concentration (ppm) 17(a) SRM-3 72 17(b) RM-1 25

Example 18

This example shows sorption of mercury (metal) from naphtha bysulfidized red mud (a) and red mud (b). In part (a), a solution of 500ml naphtha containing 100 ppb of mercury was slurried with 10 grams ofspray dried sulfidized red mud (SRM) for 30 minutes. The resultingslurry was filtered, and the SRM filter cake was dried for 1 hour atroom temperature and analyzed for mercury by ICP.

In part (b), the procedure of part (a) was repeated using red mud (RM).

Results of parts (a) and (b) are shown in Table VI and reveal theincreased capture of mercury from naphtha by sulfidized red mud (SRM-3).

TABLE VI Mercury (ppb) 18(a) SRM-3 filtrate 46 18(b) RM-1 filtrate 21

Example 19

This example shows sorption of chromium (III) by sulfidized red mud(SRM) and red mud (RM). In part (a), ten grams of SRM was slurried 30minutes with 1 kg demineralized water containing 2.240 ppm chromium III.The slurry was filtered and the filtrate analyzed for chromium by EPA200.9 method.

In part (b), the procedure of part (a) was repeated using 2.240 ppmchromium III and red mud (RM). The results are shown in Table VII below.

TABLE VII Chromium III (ppm) Control 2.240 19(a) SRM-3 filtrate 0.00519(b) RM-1 filtrate 0.018

Results shown in Table VII demonstrate improved sorption of Chromium IIIby SRM-3 compared to RM-1.

Example 20

This example illustrates sorption of cobalt (II) by sulfidized red mud(SRM) and by red mud (RM). The procedures of Examples 19 (a) and (b)were repeated using 2.75 ppm of cobalt II. The results are shown inTable VIII below.

TABLE VIII Cobalt II (ppm) Control 2.75 20(a) SRM-3 filtrate 0.013 20(b)RM-1 filtrate 0.046

Results in Table VIII show that SRM-3 has greater affinity for cobalt IIthan RM-1, with the filtrate from SRM-3 containing less than ⅓ of cobaltII than that contained in the filtrate from RM-1.

Example 21

This example shows sorption of nickel (II) by sulfidized Red Mud (SRM)and by red mud (RM). The procedures of Examples 15(a) and (b) wererepeated using 1.13 ppm nickel (II). The results are shown in Table IXbelow.

TABLE IX Nickel II (ppm) Control 1.13 21(a) SRM-3 filtrate 0.056 21(b)RM-1 filtrate 0.009

The results show nickel removal by SRM-3 was less efficient than byRM-1.

Example 22

This example illustrates sorption of copper (II) by sulfidized red mud(SRM-3) and by red mud (RM-1). The procedures of Examples 19 (a) and (b)were repeated using 1.550 ppm, 6.250 ppm, and 30.500 ppm copper (II).The results are shown in Table X below.

TABLE X Copper II (ppm) Control A 1.550 22(a) SRM-3 filtrate <0.00422(b) RM-1 filtrate 0.028 Control B 6.250 22(c) SRM-3 filtrate 0.03822(d) RM-1 filtrate 0.054 Control C 30.500 22(e) SRM-3 filtrate 0.04022(f) RM-1 filtrate 0.073

The results show a clear advantage of SRM-3 over RM-1 for copper removalover a 15-fold range of copper concentrations.

Example 23

This example shows sorption of zinc (II) by sulfidized red mud (SRM) andby red mud (RM). The procedures for Examples 15(a) and (b) were repeatedusing 1.850 ppm zinc (II) and 2.380 ppm zinc (II). The results are shownin Table XI below.

TABLE XI Zinc II (ppm) Control A 1.850 23(a) SRM-3 filtrate 0.009 23(b)RM-1 filtrate 0.035 Control B 2.380 23(c) SRM-3 filtrate 0.022 23(d)RM-1 filtrate 0.103

The results show SRM-3 is superior to RM-1 for zinc removal and yieldsfiltrates with about one-fourth the concentration of zinc.

Example 24

This example illustrates sorption of silver (I) by sulfidized red mud(SRM). The procedure of Example 15(a) was repeated using 3.15 ppm silver(I). The results are shown in Table XII below.

TABLE XII Silver I (ppm) Control 3.15 24(a) SRM-3 filtrate N.D.

The results demonstrate that SRM-3 is a good sorbent for silver ion.

Example 25

This example shows sorption of gold I by sulfidized red mud (SRM). Theprocedure of Example 19 (a) was repeated using 0.703 ppm gold III. Theresults are shown in Table XIII below.

TABLE XIII Gold III (ppm) Control 0.703 25(a) SRM-3 filtrate 0.227

The results demonstrate that SRM-3 is a good sorbent for gold (III).

Example 26

This example illustrates sorption of cadmium II by sulfidized red mud(SRM) and by red mud (RM). The procedures of Examples 19 (a) and (b)were repeated using 1.850 ppm cadmium. The results are shown in TableXIV below.

TABLE XIV Cadmium II (ppm) Control 1.850 26(a) SRM-3 filtrate 0.00926(b) RM-1 filtrate 0.035

The results show that SRM-3 is significantly more efficient in removingcadmium II from water than is RM-1.

Example 27

This example shows sorption of lead ion ⁺2 by sulfidized red mud (SRM)and by red mud (RM). The procedures of Examples 19 (a) and (b) wererepeated using 2 ppm lead ion (⁺2). The results are shown in Table XVbelow.

TABLE XV Lead II (ppm) Control 2.0 27(a) SRM-3 filtrate 0.007 27(b) RM-1filtrate 0.058

The results show that SRM-3 reduced lead content to about one-eighth ofthe content achieved by RM-1. The lead content of the SRM filtrate (7ppb) met drinking water standards (currently 15 ppb).

Example 28

This example shows sorption of selenium by sulfidized red mud (SRM) andred mud (RM). The procedures of Examples 19 (a) and (b) were repeatedusing 2.5 ppm selenium. The results are shown in Table XVI below.

TABLE XVI Selenium (ppm) Control 2.5 28(a) SRM-3 filtrate 0.24 28(b)RM-1 filtrate 2.10

The results show that SRM-3 reduced Se by about 90% while RM-1 onlyreduced Se by about 16%.

Example 29

This example illustrates sorption of uranium by sulfidized red mud(SRM-3) and red mud (RM-1). The procedures of Examples 19(a) and (b)were repeated using a Uranium Atomic Absorption Standard Solutioncontaining 1000 micrograms of U (as uranyl nitrate—UO₂(NO₃)₂) and madeup in varying concentrations (1.13, 10.1, and 38.0 ppm), and thentreated with sulfidized red mud (SRM-3) and red mud (RM-1). In addition,a third test was performed on each uranium solution using a mixture of 5g sulfidized red mud (SRM-3) and 5 g red mud (RM-1). The results areshown in Table XVII below.

TABLE XVII Uranium (ppm) Control A 1.13 29(a) SRM-3 filtrate 0.040 29(b)RM-1 filtrate 0.074 29(c) RM-1/SRM-3 0.031 Control B 10.1 29(d) SRM-3filtrate 0.494 29(e) RM-1 filtrate 2.450 29(f) SRM-3/RM-1 filtrate 1.610Control C 38.0 29(g) SRM-3 filtrate 3.950 29(h) RM-1 filtrate 6.90029(i) SRM-3/RM-1 filtrate 4.660

The data in Table XVII (29(f)-(i)) confirm that sulfidized red mud issignificantly more efficient for extraction of uranium than is red mud.Moreover, combinations of sulfidized red mud and red mud (1:1) are moreeffective than red mud alone. The combination of SRM and RM allows thecomplimentary extraction of elements while eliminating the leaching ofother elements from RM.

Table XVIII below summarize the results of Examples 19-27. The lastcolumn indicates the amount (in wt %) of the target material that wasremoved by SRM.

TABLE XVIII Control RM SRM % Removed Example Element (ppm) (ppm) (ppm)by SRM 19 Chromium III 2.240 0.018 0.005 99.997 20 Copper II 1.550 0.028<0.004 99.997 Copper II 6.250 0.054 0.038 99.993 Copper II 30.500 0.0730.040 99.999 21 Zinc II 1.850 0.035 0.009 99.995 Zinc II 2.380 0.1030.022 99.990 22 Silver I 3.15 ND* ND 99.999 23 Gold I 0.703 ND  0.22767.7 24 Cadmium II 1.850 0.035 0.009 99.995 25 Lead II 2.0 0.058 0.00799.996 26 Selenium 2.5 2.1 0.24 99.904 27 Uranium II 1.13 0.074 0.0499.964 Uranium II 10.1 2.45 0.494 99.951 Uranium II 38.0 6.90 3.9599.896 *ND = not detectable

Example 30

This example compares SRM and RM for sorption of As, Co, Mn, and Sr. Theprocedure of Example 9 was repeated using solutions of arsenic (III),arsenic (V), cobalt II, manganese (II), and strontium (I), with resultssummarized in Table XIX.

TABLE XIX Control RM-1 % SRM-3 % Element (ppm) ppm Removed Ppm RemovedArsenic III 0.60 0.11 81.7 0.36 60.0 Arsenic V 1.60 0.21 87.8 1.15 72.0Cobalt II 2.75 0.013 99.5 0.046 98.3 Manganese II 1.63 0.135 91.7 0.54866.4 2.10 0.72 65.7 0.792 37.7 Strontium II 1.90 0.10 94.7 1.10 42.1 9.00.08 99.1 4.60 48.9 27.0 0.19 99.3 11.0 59.2

These experiments reveal that the efficiency of red Mud (RM-1) issignificantly better than SRM-3 in the case of As (III), As (V), Mn(II), and Sr (II). However, the use of red mud as a sorbent is limitedby the leaching of undesirable elements which can and have causedserious problems. Use of sulfidized red mud in combination with red mudallows utilization of the latter because sulfidized red mud sorbsundesirable leaching of extraneous metals from red mud itself.

Example 31

This example shows sorption of Hg (II) by various sulfidized red muds,as summarized in Table XX below.

TABLE XX Concentration of Hg (II) in Leachate (ppm) SRM-4 SRM-5 SRM-6SRM-3 Concentration of Hg (II) 5% % 5% % 5% % H₂S % in Original solution(ppm) (NH₄)₂S Removed Na₂S Removed CaS_(x) Removed pressure Removed 4.50.001 100 0.449 90.0 0.005 99.9 0.004 99.9 19.6 0.0229 99.9 15.4 21.43.16 83.8 0.02 99.9

Each of SRM-3, -4, and -6 gave excellent sorption results from solutionsof Hg (II) at two concentration (4.5 ppm and 19.6 ppm). It issignificant that SRM-4 reduced Hg to 1 ppb, thus meeting currentdrinking water standards (2 ppb maximum). SRM-5 made form red mud bytreatment with Na₂S was much less efficient. Ammonium sulfide treatment(SRM-4) was the most effective sorbent despite the fact it had thelowest S content as shown by the analysis in Example 7.

Example 32

This example illustrates treating mercury metal with red mud andsulfidized red mud (wet). In part (a), a mixture of 10 g mercury metal,50 g red mud, and 100 g demineralized water was rapidly mixed in aWaring Blender for 10 minutes. The aqueous slurry of red mud wasseparated from mercury in a reparatory funnel. The slurry was filtered,dried at 80° C. for 4 hours, then ground in a coffee grinder for 3minutes, and submitted for mercury analysis.

In part (b), the procedure of part (a) was repeated using SRM-2. In part(c), the procedure of part (a) was repeated using SRM-410, which wasprepared by reaction of red mud and 10% ammonium sulfide. Results forparts (a)-(c) are shown in Table XXI below.

TABLE XXI Example Reagent % Hg 32(a) RM-1/Hg 1.27 32(b) SRM-2/Hg 0.5532(c) SRM-410/Hg 1.65

The results show that sulfidized red mud SRM-410 of Example 32(c) wasabout 30% more effective than red mud (RM-1) in sorbing mercury.

Example 33

This example illustrates treating mercury metal with red mud andsulfidized red mud (dry). In part (a), a mixture of 10 g mercury metaland 50 g red mud was rapidly mixed in a Waring Blender for 10 minutes.Demineralized water (100 g) was added to the mixture and mixing in theblender resumed for 5 minutes. The aqueous slurry of red mud wasseparated from mercury in a reparatory funnel. The slurry was filtered,dried at 80° C. for 4 hours, then ground in a coffee grinder for 3minutes, and submitted for mercury analysis.

In part (b), the procedure of part (a) was repeated using SRM-2. In part(c), the procedure of part (a) was repeated using SRM-410, which wasprepared as described in Example 32 above. The results are provided inTable XXII below.

TABLE XXII Example Reagent % Hg 33(a) RM-1/Hg 1.84 33(b) SRM-2/Hg 6.3433(c) SRM-410/Hg 5.58

The results show that sulfidized red mud SRM-2 and SRM-410 sorbed overthree times as much mercury than did red mud (RM-1). The sorptionprocedure in Example 33, which used direct contact of the sulfidized redmud and mercury (without water present initially), was much moreeffective than the procedure in Example 32, which initially added waterto the mercury and sulfidized red mud.

Example 34

This example shows sorption of thorium (IV), as Th(NO₃)₄.H₂O, by RM-1and SRM-3. In part (a), 10 g of sulfidized red mud (SRM-4) was slurriedfor 30 minutes with 1 kg demineralized water containing 1 ppm thorium.The slurry was filtered and analyzed for thorium.

In part (b), the procedure of part (a) was repeated using 5 ppm thorium.In part (c), the procedure of part (a) was repeated using 10 ppmthorium. In part (d), the procedure of part (a) was repeated using 20ppm thorium. The procedures of parts (a)-(d) were then repeated usingred mud. The results are summarized in Table XXIII below.

TABLE XXIII Th (ppm) Example Control SRM-4 RM-1 34(a) 0.956 ND* 0.05134(b) 4.930 ND 0.260 34(c) 10.500 ND 0.564 34(d) 19.400 ND 0.921 *ND =not detectable

The results show that sulfidized red mud SRM-4 was very effective(essentially quantitative) for thorium sorption.

Example 35

This example compares sedimentation rates of SRM-3 and RM-1. In thecourse of tests on metal sorption from aqueous solutions by sulfidizedred mud and red mud, it was found that in all cases, sulfidized red mudexhibited significantly faster filtration rates than red mud. Red mud isvery hydrophilic but conversion of red mud to sulfidized red mudtransforms it to a lyophobic particle which is more readily dewatered.The unexpected improvement of dewatering behavior is shown in thefollowing experiment:

A dispersion of 50 grams of RM-1 in 500 ml demineralized water wasprepared by rapid mixing in a Waring Blender for 10 minutes. Theexperiment was repeated using 50 grams of SRM-3 in 500 ml demineralizedwater.

Both freshly prepared slurries were allowed to settle undisturbed atambient temperature (25° C.) for a period of 72 hours. After 72 hours,the RM-1 dispersions had settled to give a clear supernatant layer ofonly 1 cm. The remaining slurry consisted of dispersed RM-1 with novisible sediment.

During the 72 hour period, the SRM-3 slurry completely settled tofurnish a sedimentary layer about 1 cm deep and a clear supernatantlayer 11.5 cm above the sediment.

These results clearly show the total alteration of surface chemistry anddewatering characteristics of red mud by relatively small degrees ofsulfidation.

Example 36

Five kilograms of sulfidized red mud from Example 4 was mixed with threekilograms of water containing 50 grams of sodium silicate in a rotatingspherical pelletizer (candy pan) for 30 minutes and then screened toreject and recycle plus 6 mm and minus 3 mm particles. The resultingpellets were dried for four hours at 110° C. The pellets were packed ina filter bed 60 cm deep and used to filter dilute solutions of heavymetals.

While particular embodiments of the present invention have beendescribed and illustrated, it should be understood that the invention isnot limited thereto since modifications may be made by persons skilledin the art. The present application contemplates any and allmodifications that fall within the spirit and scope of the underlyinginvention disclosed and claimed herein.

1. A process of treating a medium comprising (i) flue gas from coal- oroil-fired power plants, (ii) a hydrocarbon stream containing heavy metalions, (iii) a dry or wet mixture containing heavy metal ions, or (iv)elemental mercury, the process comprising contacting the medium with asorbent comprising sulfidized red mud.
 2. The process of claim 1,wherein the medium comprises flue gas from coal- or oil-fired powerplants, and the medium is flowed through a dry or wet scrubbercontaining said sorbent comprising sulfidized red mud.